Sensory and autonomic small nerve fiber function was studied in 142 type I diabetics and 45 control subjects. Thermal sensitivity (TS), hot pain sensitivity, and the activity of sweat glands (SGs) were quantitated on the dorsum of the hand and the foot. TS was abnormal in 86% of patients in the foot and 66% in the hand. TS was more sensitive than the hot pain threshold, the number of pilocarpine activated SGs, or the amount of sweat secreted, all of which were abnormal in about 50% of patients for the foot and less than 25% in the hand. Thermal and sweating tests correlated significantly with the scores of abnormal temperature and pinprick sensation obtained by physical examination but not with the duration of the diabetes. TS was also correlated with motor and sensory nerve conduction parameters, but SG number was not. The results indicate that diabetic neuropathy has a variable presentation in different types of nerve fibers.